An electric double layer capacitor includes a capacitor element having a pair of polarizable electrodes and a separator. Each of the polarizable electrodes includes a current collector of an aluminum foil or the like and activated carbon-based electrode layers formed on the current collector. The polarizable electrodes are wound with the separator interposed therebetween to form the capacitor element. The capacitor element is then impregnated with an electrolyte solution to form the electric double layer capacitor. In order to reduce its deterioration due to electrolysis, the electrolyte solution has an electrolyte solute dissolved therein, the electrolyte solute containing anions which contain a halogen such as fluorine. Such an electric double layer capacitor is disclosed in Patent Document 1, for example.
If the electrolyte solution generates fluorine ions, which are decomposition products of the anion, or contains fluorine ions as impurities, the fluorine ions react with the aluminum used as the current collector. This causes the formation of a passive film of aluminum fluoride between the current collector and the polarizable electrode layers. The passive film causes an increase in the interface resistance between the current collector and the polarizable electrode layers and hence the resistance of the electric double layer capacitor is increased.
Patent Document 1: Japanese Patent Unexamined Publication No. 2002-260966